1. Field of the Invention
The present invention relates to an improvement of a system for sensing a magnetic field with use of a superconductor magneto-resistive element of superconductive material, and in particular to an improvement of measurement accuracy in a high sensitive superconductor magnetic field sensor system having a bias magnetic field applied to the magneto-resistive element constructed of the superconductive material having weak coupling grain boundaries.
2. Description of the Prior Art
Conventionally, a magnetic sensor utilizing a hall effect of magneto-resistive effect in a semiconductor or in a magnetic material is widely used for sensing or measuring a magnetic field. In particular, there is utilized a magneto-resistive element using a shape effect in a semiconductor with high speed electron mobility such as InSb and InAs, or using an orientation effect in a ferro magnetic metal such as Fe-Ni, Co-Ni.
In addition, there has been developed a method for sensing and measuring a feeble magnetic field utilizing a magneto-resistive effect of a superconductor oxide made of superconductive material with grain boundaries weakly bonded.
In the conventional magneto-resistive element using a semiconductive material or magnetic material, since the sensitivity of measuring a magnetic field is low when the magnetic field is in a low or weak range, a bias magnetic field is applied to the weak magnetic field to be measured using such as a permanent magnet so as to transfer the low range of the magnetic field to a high sensitive range to be measured by the magneto-resistive element. Even in this case, however, it has been difficult to measure such a feeble magnetic field with good accuracy.
In a method of measuring a magnetic field using a superconductor magneto-resistive element, when there is applied a magnetic field of a predetermined intensity greater than a threshold value determined depending on the value of the bias current flowing through the element, the superconductive condition of the element is broken and transited to a normal conductive condition. Therefore, in the region greater than the threshold at this transition point, the resistance value of the element is abruptly increased so as to generate the output voltage proportional to the resistance value, thereby obtaining a high sensitive measurement of a magnetic field in this region thereof.
In such a superconductor magneto-resistive element, however, there occurs a fluctuation phenomenon in a low frequency range smaller than 10 hertzs of a fluctuation of an output of the element. Therefore, it has been difficult to measure a feeble magnetic field applying a D.C. bias magnetic field or A.C. bias magnetic field less than 10 Hz. Moreover, there occurs a change in the output voltage value of the element due to a temperature fluctuation inherent in the superconductor magneto-resistive element when the element is cooled to hold the superconductive state, and the change in the output of the element due to the temperature fluctuation can not be corrected.